Modern disc drives typically comprise one or more discs that are coated with a magnetizable medium and mounted on a hub of a spindle motor for rotation at a constant high speed. Information is written to and read from nominally circular, concentric data tracks on the discs through the use of a read/write head mounted to a movable actuator assembly positioned adjacent the discs. The actuator assembly typically includes a plurality of actuator arms that extend over the discs, with one or more flexures extending from each of the actuator arms. Mounted at the distal end of each of the flexures is the read/write head, including a write transducer for writing information to the tracks and a read transducer for reading information from the tracks when the read write head is positioned over the desired track.
To move the heads over the desired track, the actuator assembly typically includes a voice coil motor (VCM), which includes a coil attached to the actuator assembly, as well as one or more permanent magnets that establish a magnetic field in which the coil is immersed. The controlled application of current to the coil causes magnetic interaction between the permanent magnets and, as a result, the coil moves in accordance with the well known Lorentz relationship. As the coil moves, the actuator assembly pivots about a bearing shaft assembly, and the heads are caused to move across the surfaces of the discs.
Each of the concentric data tracks on a disc typically includes a number of data sectors for recording used data. In addition, special servo information is typically included in each track to assist in determine the position of the read/write head. The servo information is typically written in a plurality of servo sectors that are angularly spaced from one another and interspersed between data sectors around each track of each disk. Each servo sector typically includes a track identification (ID) field and a group of servo bursts. To position a read or write transducer over a desired track, a servo control system uses the track ID field as a control input and calculates and applies an appropriate current to the coil of the voice coil motor to move the transducer toward the desired track during a coarse “seek” mode. Once the transducer is generally over the desired track, the servo control system uses the servo bursts to keep the transducer over that track in a fine “track follow” mode. The read transducer generally reads the servo bursts to produce a position error signal (PES) that is indicative of the position of the read element, relative to a predetermined radial position on the track.
To ensure reliable storage and retrieval operations by the disc drive, the user data is typically encoded with an error correction codes (ECC) before being written to the disc. The ECC is used to detect and correct up to a selected number of errors in the retrieved sequence of data. Occasionally a disc drive will read erroneous data from a data sector that cannot be corrected by the ECC. These type of uncorrectable errors (read errors) are typically detected by an error correction code circuit (ECC) in the disc drive, which may perform both error detection and error correction upon the data read from the data sectors (using, for example Reed-Solomon codes). If no uncorrectable errors are present, the read data is output to the user. However, if the ECC circuit is not able to correct the error(s), a read error is declared, and appropriate remedial actions are undertaken.
Read errors may occur for a number of different reasons. For example, read errors may be caused by a defect, such as an asperity on the disc at or near the location on the disc where the desired data has been written. Read errors may also occur due to the inaccurate or erroneous positioning of the read transducer relative to the data that is to be read. This may be due either to incorrectly positioning the read transducer at the time the read operation is performed or, more commonly, as a result of an off-track write, where the data is inadvertently written at an incorrect radial location relative to the center of the track.
When a read error occurs during a read operation in a disc drive, a number of different read error recovery techniques may be employed to correct the error. For example, in the simplest case, after a read error has occurred with respect to a data sector, the read transducer is maintained at its current location and the data sector is simply read again the next time the data sector rotates beneath the read transducer. That is, a read retry operation is performed with respect to the data sector. Another technique that may be employed involves performing a read retry operation while adjusting various read processing parameters. Yet another technique that may be employed involves moving the transducer a slight distance from its current location (performing an offset operation) and then performing a read retry operation. Performing track offset and retry operations together is a particularly useful technique in cases where the data associated with the erroneous read operation has been inadvertently written off track.
There are a number of processes that may be used to perform offset/read retry operations. In accordance with one method, after a read error is detected with respect to a given data sector on a track, the read transducer is offset a predetermined distance from the center of the track, for example 4% of track pitch (4% TP), and a first read retry operation is performed with respect to the given sector. If the first read retry reads the given sector without a read error, the process ends. If, however, the first read retry operation produces a read error, the read transducer is then offset 4% TP from track center on the opposite side of the track center from the first offset, and a second read retry operation is performed. If the second read retry operation produces an read error, the read transducer is then offset 8% TP on the opposite side of the track, and a third read retry operation is performed. If the third read retry operation produces an read error, the read transducer is then offset 8% TP on the opposite side of the track center from the first offset, and another read retry operation is performed. This same process of moving the read transducer back and forth to opposite sides of the track center in ever increasing increments of track pitch is continued until either a read retry successfully reads the data, or until a predetermined number of retry operations have been performed.
There are a number of drawbacks associated with this offset/read retry process. First, sweeping the read transducer back and forth across the track center to perform the read retry operation in this manner can result in unwanted resonance in the mechanical components of the disc drive. The mechanical resonance may induce noise into the servo positioning system, thus making it increasingly difficult to position the read transducer accurately. Additionally, the mechanical resonance may cause undesirable audible vibrations to occur in the disc drive. Secondly, since a full rotation of the disc must occur for each retry operation, a significant amount of rotational latency may be incurred if a successful read retry operation is not achieved early in the retry process.
Another offset/read retry process that may be used involves first performing a predetermined number of offset/retry operations, on one side of the track center, and then performing the same number of offset/retry operations on the opposite side of the track center. In accordance with this process, each offset moves the read transducer an identical predetermined distance from its previous radial position. For example, a first offset may move the transducer 4% TP from the track center to a first radial location, the next offset will move the transducer 4% TP from the first radial location to 8% TP from the track center, and so on. After a predetermined number off offset/retry operations have been performed on one side of the track, the transducer is moved to other side of the track and the process is repeated. As with the previously described offset/retry process, anytime during the process when a non-erroneous retry operation is performed, the process ends.
As will be recognized, since this offset/read retry process does not sweep the read transducer back and forth across the track center each time a read retry operation is performed, the unwanted resonance associated with the first offset/read retry process is avoided. Additionally, this offset/read retry process will typically take half the time to complete than the previously described offset/read retry process in the case where a successful read retry operation occurs on side of track where the process began. That is, this offset/read retry process will be faster than the previously described offset/read retry process when the process begins on the “correct” side of track center. However, the other 50% of the time, where the process does not begin on the “correct” side of track center, and in the case where a successful read retry operation is never performed, this offset/read retry process may take the same amount of time as the previously described offset/read retry process.
Accordingly there is a need for a offset/read retry process that quickly determines the “correct” side of the track center on which a successful read retry operation may be achieved, and which minimizes the time need to reach a position on the “correct” side of the track center where the successful read retry achieve is achieved.